1. Field of the Invention
The invention relates to a composition for producing foams.
2. Background Art
The chemical structure of polysiloxane-containing (silicone-containing) elastomers, thermoplastic elastomers, and resins, does not give them microwave activity. They are therefore not heatable or crosslinkable (vulcanizable) by microwave radiation—without specific additives or fillers. Use of fillers having microwave activity, e.g., magnetite (cf., for example, DE 10 2004 053 310 A1, DE 10 2004 053 309 B3) can achieve rapid heating of the rubber or of the crude compositions, and can therefore achieve crosslinking to give the elastomer or resin, with the aid of suitable vulcanization additives (e.g., peroxides or noble metal catalysts).
The use of microwaves for the heating of plastics is well known in the thermoplastics field (e.g., EP 0 351 544, DE 39 23 913 A1), and is also used in the field of organic elastomers (for example DE 692 07 040 T2), or generally in the polymer field (for example DE 689 18 648 T2, DE 43 34 453 C2)). However, for the abovementioned reasons, this method has not been used, or has not been used widely, in the field of polysiloxane plastics (U.S. Pat. Nos. 4,980,384, 4,460,713). None of the compounds described in the prior art can give a homogeneous foam.
Traditional foaming of, for example, siloxane-containing polymers uses external heating elements of very varied type to introduce heat from outside into the formulation to be foamed, this heating being needed not only for vulcanization but also to activate (decompose), for example, metastable foam-blowing agents. The following disadvantages result for the foam part to be produced:                the problem of a critical balance of crosslinking and expansion (e.g., little processing latitude, and little opportunity for control);        a surface problem, e.g., a risk of pores breaking through the surface skin if these have not been adequately crosslinked throughout;        the problem of inhomogeneous foam structure, e.g., production of large and small pores, and also of regions without pore formation;        the problem of dimensional stability, e.g., lack of reproducible shape and lack of exact adjustment of part dimensions, due inter alia to the abovementioned problems;        the problem of restricted heat transfer, e.g., the foam being produced is even less efficient than the crude composition in conducting heat from the outside to the inside, thus expansion shortfalls and/or vulcanization shortfalls occur in the interior.These factors give rise to the disadvantages known to the person skilled in the art in the traditional heat-induced foaming of siloxane-containing compositions:        the entire process is metastable to unstable;        there is but restricted selection of processing methods when comparison is made with conventional thermoplastics, elastomers, etc.;        a restricted selection of processing parameters;        production of parts with high dimensional accuracy or of parts which always have the same homogeneous foam structure is not possible;        complicated and expensive equipment is needed;        the cost for design and development of foam parts is very high.        